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I. INTRODUCTION

The lanthanides have been extensively studied as luminescent ions when incorporated in crystalline host materials.1"17 There are several features of the lanthanides that make them unique as luminescent dopants. Electrons of the partially filled 4/ shell, which give rise to many of the electronic states from which emission is observed, are well shielded by the outer electron shells (5s and 5p). Consequently, the 4/ electronic energy levels are not strongly perturbed by their environment, and the spectrum of lanthanide-doped crystals closely resembles that of the free ion.1 In addition, electric-dipole transitions are forbidden between electronic states arising from the 4/ electron configurations since all such states are of the same parity.18 As a result, excited state lifetimes are long and correspond to magnetic-dipole transitions or electric-dipole transitions induced by parity breaking interactions with the crystal lattice vibrations.2 Finally, electronic states that involve higher energy orbitals (i.e., 5d) or those that involve charge transfer from the adjacent lattice atoms usually lie in the ultraviolet region of the spectrum. These outer orbital electronic states are affected to a much greater extent by lattice interactions which result in broad spectral features in the ultraviolet region of the spectrum. These high-lying states provide a convenient excitation route to populate the lower lying electronic levels that readily emit in the visible spectral region. Practical applications of the lanthanide luminescent materials include solid-state laser oscillators and amplifiers19 and phosphors.20 Laser applications require growth of large lanthanide-doped single crystals whereas phosphor applications generally require thin films. Cathode ray tubes, for example, use films formed J. Mater. Res., Vol. 5, No. 7, Jul 1990

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from powdered materials applied to a substrate with a suitable binder. For more stringent applications, such as head-up displays, films with smoother surface morphology, better adhesion, and the ability to withstand higher temperatures are required.21 Chemical vapor deposition is a technique which can produce thin films with minimal surface roughness and strong adhesion to the substrate even at high temperatures. Atmospheric pressure MOCVD of Y2O3:Eu has previously been demonstrated for applications in cathodoluminescence.22 We have extended the technique to the 1 Torr pressure region and report that Y2O3:Eu films produced by lowpressure MOCVD have excellent photoluminescent properties and are stable to at least 1200 °C. We have also studied the photoluminescent lifetimes of the Eu+3 5 £>o -»• 7F2 and 5£>i -> 7Fi transitions in low-pressure MOCVD films as a function of temperature from room temperature to 1000 °C following pulsed excimer laser excitation at 248 nm. II. EXPERIMENTAL

The hot-wall MOCVD reactor chamber is a 5 cm diam fused silica cylindrical tube enclosed in a one-zone tube furnace. Fused silica, sapphire, or pol